TNP modified mouse lymphoma cells bearing large amounts of H-2k antigens were used to prepare plasma membrane vesicles by nitrogen decompression and centrifugation. Such membrane vesicles were mixed with allogeneic mouse T lymphocytes which had been activated by a 5-day in vitro culture with irradiated H-2k spleen cells. After incubation with vesicles the cells were washed, stained with FITC anti-TNP and examined by fluorescene microscopy and the FACS. Specific binding of membrane vesicles was observed. In order to identify the antigen recognized on the bound vesicles, monoclonal anti-H-2k antibody was labelled with FITC and 125I and its binding to various lymphocytes and tumor cells studied. A pseudo saturation of antibody binding was observed which showed approximately 10 to the 5th power antibody molecules bound per spleen cell of the appropriate H-2 specificity. A lymphoid tumor, RDM-4, was found to have over 10 times this level of antibody binding.